1. Field of the Invention
This invention relates to a method of and apparatus for inspecting slight defects in a photomask pattern.
2. Description of the Prior Art
A photomask 1 is used in the production of semiconductor integrated circuits. As shown in FIG. 1, the photomask 1 has a transparent base 2 on which, for example, two chips 3 each size of which is 10 mm.times.20 mm are formed. A circuit pattern 4 of a light intercepting film made of chromium (Cr) or the like is formed in the chips 3 in high density. The width of the circuit pattern 4 is, for example, 1 .mu.m to 3 .mu.m.
As shown enlargedly in FIG. 2, cases occur in which a part of the circuit pattern 4 has defects, such as a pin hole 5 or a projection 8, or has flaws, such as a crack 6 or a nick 7, or has foreign substances. If exposure is performed using such a defective photomask 1, a circuit pattern different from a predetermined circuit pattern 4 is formed on a semiconductor substrate (i.e., a wafer). In other fords, a semiconductor integrated circuit having pattern defects is formed. For this reason, an inspection of whether the formed photomask 1 has defects is beforehand undergone.
There are various kinds of methods of inspecting defects in the pattern of the photomask 1. Typically, an adjacent-pattern comparison method and a design-data comparison method are well known.
1 Adjacent-Pattern Comparison Method
According to this method, two adjacent chips 3 are compared with each other and, when disagreements therebetween are found, it is judged that defects exist. This method is followed on the supposition that there is little probability that two adjacent chips 3 have the same defects in the same circuit patterns of the chips 3.
2 Design-Data Comparison Method
According to this method, a circuit pattern is observed by a defect inspection apparatus, and the observed positions are compared with design-data corresponding to the positions.
These defect inspection methods 1 and 2 are properly used depending on purposes and uses.
FIG. 3 shows an example of the apparatus for inspecting defects in the pattern of the photomask 1. This inspection apparatus comprises a data processing system which includes a CPU 10, a magnetic disk unit 11, a magnetic tape unit 12, a floppy disk drive unit 13, a console CRT 14, a pattern monitor 15, a magnetic card unit 16, a miniprinter 17, an RS-232C adapter 18, and the like, a detective optical system which includes an autoloader control circuit 19, a table control circuit 20, an X-motor M1, a Y-motor M2, a .theta.-motor M3, an autofocus control circuit 21, a piezo-element 21a, a positioning circuit 22, a control circuit 22' of, for example, a laser length measuring system, a bit developing circuit 23, a pattern comparative inspection circuit such as a data comparison circuit 24, an autoloader 25 accommodating various kinds of photomasks 1, an illumination light source 26, an illumination field diaphragm 27, a condenser lens 28, an X-Y table 29, an objective lens 30, a photodiode array 31, a sensor circuit 32, and the like, and an observing scope which includes reflection mirrors 33, 34, an eyepiece 35, and the like (see a reference entitled "Mask Defect Inspection Method by Database Comparison with 0.25.about.0.35 .mu.m Sensitivity", in Jpn. J. App. Phys, Vol 33(1994)7156). As shown in FIG. 4(a), the width of about 300 .mu.m of the photomask 1 is observed by the photodiode array 31. The photodiode array 31 is disposed at a position where the circuit pattern is imaged. The photomask 1 is mounted on the X-Y table 29 and is illuminated with light from the illumination light source 26.
As shown in FIG. 4(b), the X-Y table 29 is transferred in the direction of arrow A1 at intervals of a predetermined pitch p. When measurements in the direction of arrow A1 are completed, the X-Y table 29 is transferred by the width W in the direction of arrow A2 and thereafter the X-Y table 29 is transferred at intervals of the predetermined pitch p in the direction of arrow A3. In the same manner, the X-Y table 29 is transferred successively in the directions of arrows A4, A5 . . . so as to inspect the whole range of the photomask 1.
The autofocus control circuit 21 drives autofocusably the objective lens 30 in the axial direction of the objective lens 30 so as to keep a distance between the objective lens 30 and the photodiode array 31 constant, and thereby accurate data can be obtained. The .theta.-motor M3 controls the X-Y table 29 to keep the photomask 1 parallel to the photodiode array 31.
Graphic data is beforehand stored as a circuit pattern in the magnetic disk unit 11. The circuit pattern 4 of the photomask 1 is projected enlargedly onto the photodiode array 31 by means of the objective lens 30, and an image of the circuit pattern 4 is formed on the photodiode array 31. The image of the circuit pattern 4 is photoelectrically transferred by the photodiode array 31 and is output to the sensor circuit 32 in the form of measured data. The measured data is converted from an analog signal to a digital signal and is input to a first input terminal of the data comparison circuit 24.
On the other hand, the graphic data is transmitted to the bit developing circuit 23 in accordance with a detected output of she positioning circuit 22. The graphic data is converted into a binary number system by means of the bit developing circuit 23 and is transmitted to a second input terminal of the data comparison circuit 24. The output of the positioning circuit 22 is input to a third input terminal of the data comparison circuit 24. The data comparison circuit 24 processes the binary bit pattern data through proper filters and thereby converts the binary bit pattern data into a multivalue system.
The reason why the binary bit pattern data is processed through the proper filters is that the measured data is being filtered by the resolution characteristic of the objective lens 30 and the aperture effect of the photodiode array 31.
Data in an observed position is compared with data in a corresponding position of pattern design data in accordance with a predetermined algorithm by means of the data comparison circuit 24. Thereby, disagreeing positions between the design data and the measured data are regarded as defects. In this type of pattern comparative defect inspection, in order to detect slight defects, the resolution of an optical system of an inspection means is enhanced, the algorithm for comparison is improved, or the processing of measured signals is improved.
However, the detection sensitivity to pattern defects largely depends upon the kinds of the defects. Especially, if a pattern defect of the circuit pattern 4 is a pin hole as shown in FIG. 2, it is difficult to detect it, and it is almost impossible to detect the defect of a pin-hole less then 0.35 .mu.m in diameter.
In recent years, a phase shift type of photomask shown in FIG. 6(a) has been used instead of a conventional amplitude type shown in FIG. 5(a). In the amplitude type of photomask, illumination light P1 is completely intercepted by light intercepting portions 36 made of chromium (Cr), as shown in FIG. 5(a). The illumination light P1 which has passed only through light transmitting portions 37 is guided to the photodiode array 31, and a circuit pattern is then imaged on the photodiode array 31 in accordance with the amplitude intensity of light. The luminous intensity distribution of a circuit pattern image at an imaging position 38 is shown in FIG. 5(b), where reference numeral 36' denotes a position of an intercepted image corresponding to the light intercepting portions 36, reference numeral 37' denotes a position of a transmitted image corresponding to the light transmitting portions 37, and reference numeral 39 denotes a luminous intensity distribution of the circuit pattern image at the imaging position 38.
In the conventional amplitude type of photomask 1, in order to enhance the detection sensitivity to slight defects in the circuit pattern 4, the light amplitude intensity of the circuit pattern image of the photomask 1 which is formed on the photodiode array 31 is heightened to the utmost. In other words, in order to heighten the resolution, the wavelength .lambda. of the illumination light P1 with which the photomask 1 is illuminated is shortened, and the numerical aperture NA of the objective lens 30 is enlarged. This is based on the optical theory that, if illuminating conditions are fixed, the optical intensity of an image becomes larger as the value .lambda./NA becomes smaller.
The photomask 1 which has been regarded as having no defects in the circuit pattern 4 is attached to an exposure unit. The circuit pattern 4 is then imaged on a wafer by the illumination light P1 of the exposure unit having an objective lens with a large numerical aperture NA. However, it is unallowable to make the value .lambda./NA smaller than a predetermined value, for the following reason.
A resist serving as a photosensitive agent is applied to the wafer. The film thickness of the resist is 1 .mu.m and over, as a result of considering the etching of a ground after exposure. A depth of focus equal to or larger than 1 .mu.m is required to, with respect to the direction of the film thickness, expose the resist to light while keeping the contours of the image clear.
However, the depth of focus, the wavelength .lambda., and the numerical aperture NA have a relationship to each other in that the depth of focus becomes smaller in proportion to .lambda./(NA).sup.2. Especially, the numerical aperture NA contributes to the depth of focus by the square of the numerical aperture NA. The limited value of the depth of focus is approximately 0.6.mu.m. Thus, the conventional exposure method is limited in enhancing the resolution of a circuit pattern image.
Consequently, the phase-shift photomask 40 (e.g., attenuated photomask) shown in FIGS. 6(a) and 6(b) has been used to obtain higher resolution than hitherto by the use of the conventional exposure unit.
The structure of the phase-shift photomask 40 will now be described.
As shown in FIG. 6(a), on a glass base, light intercepting portions 42 are formed which are made of a material having a higher refractive index than that of light transmitting portions 41. The light intercepting portions 42 transmit part of the illumination light P1. A phase of the part of the illumination light P1 which has passed through the light intercepting portions 42 is delayed with respect to that of the illumination light P1 which has passed through the light transmitting portions 41.
The phase difference between the illumination light P1 which has passed through the light transmitting portions 41 and the illumination light P1 which has passed through the light intercepting portions 42 causes interference therebetween. As a result, a circuit pattern image at the imaging position 38 is formed not only by the light amplitude intensity but also by the interference caused by the phase difference.
FIG. 6(b) shows the luminous intensity distribution of the circuit pattern image at the imaging position 38. In FIG. 6(b), reference numeral 41' denotes a transmission image position corresponding to the light transmitting portion 41, reference numeral 42' denotes an interception image position corresponding to the light intercepting portion 42, and reference numeral 43 denotes a distribution of the luminous intensity of the circuit pattern image at the imaging position 38.
According to the photomask 40, the minimum value .delta. of a luminous intensity distribution 43 can be made smaller than the minimum value .delta.' of a luminous intensity distribution 39. As a consequence, the contrast of the circuit pattern image having a wavelength equal to or shorter than the wavelength .lambda. of the illumination light P1 can be expected to be improved. Thus, the contours of the circuit pattern image become clear. Since the resist applied to the wafer has the property of strengthening a contrast, this effect can be heightened even more.
However, in the phase-shift photomask 40, part of light can pass through the light intercepting portions 42. This makes it more difficult to detect pattern defects, such as a pin hole with a diameter below 0.35 .mu.m, if inspection is carried out with inspection light same in wavelength as exposure light.